There is a need for thin crystalline ribbons and films of many materials such as silicon and other semiconductors. These ribbons are often very costly and difficult to produce. For example, thin wafers of monocrystalline semiconductor materials are generally produced from monocrystalline boules grown by the Czochralski technique. The preparation of the thin wafers from large crystal boules requires slicing and polishing, is a costly and time consuming technique, and inherently wastes much of the boule. Consequently, much effort has been directed toward growing thin monocrystalline ribbons that need only be scribed and broken to be used.
As reported, for example, by Chalmers, LaBelle, and Mlavsky in "Edge-Defined Film-Fed Growth," Journal of Crystal Growth, vol. 13/14, pp. 84-87, 1972, thin ribbons of semiconductors and insulators have been grown through elongated openings in a melt cover. Such growth has also been accomplished with guide posts. In these techniques the crystal is pulled in a direction perpendicular to the surface of the melt using the geometric properties of a slot in the melt cover or guide posts to establish the transverse cross section of the resulting ribbon. However, satisfactory larger ribbon widths of appropriate thickness are not favored by surface tension properties and are produced only slowly, with reduced commercial advantage. The slow growth requirement arises because the probability of grain-boundary defects is proportional to the square root of the growth velocity (pull velocity in this case), as confirmed by Pfeiffer, et al., in "Pattern Formation Resulting from Faceted Growth in Zone-melted Thin Films," Physical Review Abstracts, vol. 16, No. 10, 15 May 1985.
Ribbons of semiconductors have also been produced by pulling substantially in a horizontal plane from a melt surface, as disclosed by Bleil in U.S. Pat. Nos. 3,681,033 and 3,759,671. This latter technique produces ribbons much faster than the previous methods. However, pulling ribbons too quickly or at too great an angle from horizontal introduces grain boundaries and imperfections which degrade the performance of circuitry placed on the semiconductor surface. This fact has been described in detail in Jewett in U.S. Pat. No. 4,289,571 and Kudo in U.S. Pat. No. 4,329,195. Moreover, the necessary controls to implement the process and produce very thin crystal ribbons and film of good quality are difficult to manage and thus the commercial advantage is reduced.
It has also been proposed to horizontally produce flat ribbons by certain zone melting techniques. This has been described by Geis et al. in "Materials Research Society Symposium," vol. 13, p. 477, 1983, and Omachi et al., "Ge-Seeded Crystallization on SiO.sub.2 " in Electronics Letters, vol. 19, No. 8, Apr. 14, 1983, the latter by using a slider system with an rf heated strip heater. However, such techniques develop freezing isotherms nearly perpendicular to the pulling direction and have not been wholly satisfactory for producing thin ribbons of high quality at low cost. Such difficulties may be avoided by crystallization, according to the present invention. Ribbons of materials other than semiconductors may be similarly produced by zone melting and recrystallization as shown by this invention.
In some applications, it is desirable to prepare monocrystalline ribbons or films of semiconductor or other materials on an insulator substrates, such as semiconductor on insulator, or SOI, structures. This can be accomplished by growing the ribbon on the insulator or bonding the ribbon onto insulator material by using, for example, electrobonding techniques. The desired monocrystalline structure can be given to the ribbon material through a variety of "seeding" techniques, including beginning and maintaining the seeding process at a location away from the insulator material, thereby inducing a monocrystalline form to be propagated through the ribbon cross section.
For ribbons or SOI structures, a variety of methods for supplying the zone-melting energy have been used, including lasers and graphite heaters with energy-focusing means. Methods for inducing electrical currents in the semiconductor and other materials by exposing them to a high frequency electric field have been used to recrystallize the cylindrical boules in a particularly energy efficient manner. However, methods to induce current capacitively have only recently been applied to ribbons in a manner permitting control of the shape and size of the zone of recrystallization as disclosed by Bleil in U.S. patent application No. 816,424, filed Jan. 6, 1986. Capacitive coupling not only avoids electrode contact with the material but also permits a current gradient to be established within the melted zone. While it is generally desirable to avoid contact with the melt by foreign materials, in some applications the current gradient may be established by direct contact with electrodes of the same material as the melt or by other suitable means to cause zone recrystallization in a thin layer of the material in a manner allowing the shape and size of the zone to be controlled.